Astrometric calibration with Aladin

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Astrometric calibration with Aladin
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How to calibrate the astrometry of an
uncalibrated image step by step

• Install the Aladin standalone version
• Load your local image
• 1st solution use of an astrometrically
  calibrated image (DSS)
• 2nd solution use of an astrometrically
  calibrated point source catalogue (2MASS)
• load the DSS image/ 2MASS catalogue
• Calibrate crudely your image with the DSS image
• 1st solution plot the point sources onto your
  image
• Identify and enter image positions and
  corresponding point source positions
• 2nd solution create your own point source
  catalogue in identifying
• and entering point sources on the DSS image
• Plot the point sources onto your image
• Identify and enter image positions and
  corresponding point source positions
• Save the calibrated image

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In order to work with your own local files, you
have to install the standalone version of Aladin
from the web.(http//aladin.u-strasbg.fr/AladinJa
va?framedownloading)
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In order to load your FITS file into Aladin click
on Load. A new window pops up. Click on Local
and Browse your local file system. When you
have found your file click on SUBMIT.
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The image appears in the Aladin window together
with a warning that there is no astrometry
associated with the FITS file.We have chosen an
H band image provided by Gavazzi et al.
) The image has been downloaded from the
GOLDMine database, which is operated by
the University of Milano-Bicocca (see 2002, AA,
400, 451).
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In order to load a DSS image that is
astrometrically calibrated, click on Aladin,
type the target name, chose the image you want to
display and click on SUBMIT.

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The DSS image is loaded in a second plane and
appears in the Aladin window.
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1st solution use of an astrometrically
calibrated image. Chose a reference catalogue as
close as possible to your image wavelength (e.g.
2MASS). To load the point source catalogue for
the astrometric calibration, click on Surveys in
VizieR, chose the catalogue and click on
SUBMIT.
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The 2MASS point sources are loaded into a third
plane and appear as red crosses on the DSS image.
In order to assign first, crude coordinates to
the uncalibrated image, click on the center of
the galaxy, leave the image with the cursor and
grab the coordiantes with the cursor (written in
blue).
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Go back to the uncalibrated image, click on
prop. A new window appears. Click on New.
Again, a new window appears. Chose by
parameters. Paste the grapped coordiantes into
the corresponding field. Click on CREATE. Fill
in a first guess for the angular size of the
field.
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It is only now that you can display the 2MASS
sources on the crudely calibrated image. Click on
pixel to change the greyscale by dragging the
black arrows to the left. For an easier
calibration using only the brightes 2MASS
sources, a filter (magnitude cut of 15) can be
applied.
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The stars are now better visible. Click on
prop. A new window appears. Click on New.
Again, a new window appears. Change the upper
right panel into by matching stars, then click
on the sub-window x-y position, which then
becomes yellow.
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In order to fill in the columns, for each star
that you want to use as a reference, click first
on its x-y position on the uncalibrated image,
then click on the corresponding star of the 2MASS
catalogue (red cross). Use mglss to view the
zoomed region in the lower right corner of the
Aladin main window. At the end click on CREATE.
Zoomed view
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Now the image has a correct astrometry with
respect to the 2MASS catalogue, i.e. the 2MASS
sources (red crosses) fall onto the stars on the
image.
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2nd solution create your own star catalogue
using the DSS image. Click on tag and mark the
stars on the image.
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Change to the crudely calibrated image, click on
prop. Click on New on the new window. Change
the upper right panel into by matching stars.
Fill in the columns as before.Click on MODIFY
or CREATE.
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The correct astrometry with respect to the DSS
image is now associated with the image. One can
now check if the 2MASS point sources fall on the
stars on the image.
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To save the image click on Save. Click on
Export some planes on the new window, chose the
newly calibrated image, type the name you want
to assign to it into the corresponding field,
specify the directory where it will be saved and
click on EXPORT.
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You can also display and edit the FITS header
directly by clicking on prop in the main window
and on New on the appearing window. Change the
upper right panel in the appearing window into
by WCS header.
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Precision of the astrometric calibration
(i) When clicking on the star on an image the
program does not yet search for the source
centroide. Thus the precision of the astrometry
depends on your ability to hit the right
position of the star on the image (use mglss).
(ii) The calibrated images is written out as a
FITS file with 8 bits resolution. If your
original image had a higher resolution, it will
be saved in a degraded resolution. These items
will be improved soon.